In-line voltage contrast (VC) inspection is a powerful technique for detecting and isolating yield limiting defects in the semiconductor fabricating industry. In-line VC inspection includes scanning the wafer surface in which test structures exist with a scanning electron microscope (SEM). As the inspection proceeds, the SEM induces a charge on all electrically floating elements whereas any grounded elements remain at zero potential. This potential difference is visible to the SEM. In particular, for electron landing energies less than the second crossover of the secondary electron yield curve (approximately 2.5 keV for tungsten (W) and copper (Cu)), grounded elements appear bright whereas floating elements appear dark.
Test structures exploiting this phenomenon can be created for many yield limiting defects including metal, gate and active region shorts and opens, and via and contact opens. For example, FIGS. 1A-B, show a short (FIG. 1B) indicated by a normally floating (dark) element becoming bright, and an open (FIG. 1A) indicated when a normally bright element becomes dark. FIG. 1C shows a gate level open indicated when a normally bright element becomes dark. As shown, even if the defect causing the electrical failure is buried or extremely small, its existence is indicated by a change in the VC signal of the entire element. In addition, the exact location of an open is indicated by a change in the VC signal of the structure after the break.
E-beam inspection is a popular technique for in-line detection of yield limiting defects. One limitation is sensitivity to resistive opens and shorts. Under e-beam inspection, resistances less than about 10 Mohms look like shorts and resistances a little greater than this value look like opens. There is a resistance range where the voltage contrast appears gray, but this is relatively small.
The 10 Mohms value is used because the latest e-beam inspection tools utilize electron beam currents of up to 500 nA. The net induced current flow at the wafer surface is a fraction of this current and depends on the beam conditions. Assuming it is 30%, then the voltage induced on an electrical node with a leakage path to ground of 10 Mohms is 1.5V. Theoretically, the beam current could be changed and the induced voltage would change proportionally. In practice though, only a single voltage is used per wafer scan.
Quantifying the resistance of resistive opens is quite difficult. The ability to quantify the resistance of opens would be helpful for characterization of processes.